<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    console.log(a, fn, bar, this, '第一步');  // undefined undefined undefined Window
    var a = 10,
        fn,
        bar = function (x) {
            console.log(x, b, arguments, this, '第三步')  // 10 undefined Arguments[10] Window
            var b = 5
            fn(x + b)
        };
    fn = function (y) {
        console.log(y, c, arguments, this, '第四步')   // 15 undefined Arguments[15] Window
        var c = 5
        console.log(y + c)
    }
    console.log(a, fn, bar, this, '第二步');  // 10 function function Window
    bar(10)
</script>
</body>
</html>
